Largest iceberg on Earth runs aground

The world's largest iceberg appears to have run aground in Antarctica instead of crashing into an enormous floating tongue of ice, as predicted by previous satellite imagery.

But the iceberg remains a concern as it is starving local penguins by blocking their route to the sea and also threatening to cut off supply lines to a number of research bases in the area.

A "collision of the century" was expected on 15 January 2005 between the gigantic B15-A iceberg and the huge Drygalski ice tongue in McMurdo Sound in the Ross Sea. But satellite data from the European Space Agency and NASA reveal the clash never happened. The latest data indicates the behemoth is stranded in a bay 4 kilometres away.

"The iceberg decelerated extremely quickly before approaching the ice tongue, and the point closest to the ice tongue seemed to bounce back," says Mark Drinkwater, head of ESA's oceans and ice unit, in the Netherlands. This suggests the iceberg hit something underwater.

ESA tracked the iceberg's progress using its Envisat satellite, as its Advanced Synthetic Aperture Radar allows scientists to peer through polar clouds. The bottle-shaped iceberg is the largest floating object on Earth today, stretching 120 km in length, with an area of 2500 square kilometres.

Supply ships

Drinkwater says the colossal block could end up stuck in the sound for a year or more, with potentially serious consequences.

The iceberg is blocking sea routes to two of the major Antarctic supply bases - a New Zealand and a US base in McMurdo Sound, he says. They are currently relying on very narrow gaps in the ice for ships to squeeze through.

Furthermore, when icebergs block bays they interfere with the currents which help clear out the ice during the summer. "Instead, you get rapid development of what's called fast ice," Drinkwater told New Scientist, which is considerably thicker than normal ice.

If some of the large iceberg stays around for year or two, it might prevent any access into the bay, he warns. Until recently, B15-A had been stuck against Ross Island for 18 months.

Wildlife could also be severely affected, as the ice blockade is already posing a problem for penguins. Penguins will travel long distances across ice to reach open water, so they can fish for food. But this ends when the sea ice breaks up in the summer. If that does not happen, the adult penguins may not be able to gather enough food to support their chicks.

Relieving penguins

Images from a NASA's Aqua and Terra satellites, which use a spectroradiometer (MODIS), show some of the sea ice around the iceberg breaking up. "While the breaking ice may shorten the trip and bring relief to the penguins, it is not clear if the changing conditions will save the penguin chicks from starvation," says a NASA statement.

Previous studies have shown that icebergs can also devastate the food chain in Antarctic waters by slashing the volume of phytoplankton. This is because phytoplankton need open waters and direct sunlight to bloom.

Drinkwater speculates that the end of the iceberg closest to the 70 kilometre-long Drygalski ice tongue may stay put, but the other end may swivel round with the pressure of winds and water currents. He thinks there may be a very good reason for the iceberg's halt.

The ice tongue is unlikely to have grown so long if it was continually smashed by the numerous icebergs which break off the Ross ice shelf, he points out. "It must be inherently protected somehow by shallow shoals or sediment banks."

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The colossal iceberg, 120 kilometres long, appears to have run aground just 4 kilometres from the giant ice tongue into which it was set to collide (Image: NASA)